This invention pertains to an apparatus for forming a shadow mask for a color cathode-ray tube from a mask blank including a punch, a pad, and upper and lower dies for clamping the mask blank.
A color cathode-ray tube (CRT) employs three electron guns for emitting three electron beams which pass through a common deflection yoke, with one beam for each primary phosphor color, i.e., red, green and blue. The beams are "shadowed" by a perforated conductive mask, known as a shadow mask, so that each beam can strike but one color of a segmented cathodoluminescent screen of red, green and blue phosphors disposed close to the mask on the inside surface of a faceplate panel. At the point where the electrons from one of the guns impinge on the screen, one of the color phosphors is deposited in a spot or line that approximates the size of the mask aperture. All other parts of the phosphor screen are in the "shadow" of the phosphor mask, as far as this one gun is concerned. Thus, the position and size of the apertures in the shadow mask are important ultimately to achieve good color purity.
A conventional spherical-like shadow mask is formed from a flat mask blank by first clamping the blank around its periphery between upper and lower dies having openings therein for receiving a punch having a convex surface and a pad having a complementary concave surface. The punch and the pad are shaped with a spherical-like surface contour that provides the desired mask contour, i.e., allowance is made for springback after forming. The clamping surfaces of the upper and lower dies, adjacent the respective openings, typically have a nonplanar surface contour which is a continuation of the spherical-like surface contour of the punch surface. Consequently, when the mask blank is laid onto the lower die surface, the surface contour assumed by the blank does not match the surface contour of the die, resulting in a wrinkling of the blank as it is clamped between the upper and lower dies.
In forming the shadow mask, the wrinkles in the clamped mask blank must be stretched out to achieve a smooth surface contour. In particular, the initial portions of the mask skirt are formed while the mask blank is still clamped between the upper and lower dies, thereby stretching out the periphery of the mask to provide a smoother contour. However, in this stretching-out process, the mask apertures in the wrinkled areas stretch differently than those apertures not in wrinkled areas. This causes nonuniform stretching in the mask and, since the wrinkles are different from mask to mask, the stretching is different from mask to mask. Such nonuniform stretching of the shadow mask ultimately results in a lowering of color purity in larger size cathode-ray tubes and in high-resolution display tubes. It is desirable to have an apparatus for forming a shadow mask which avoids such a degradation in color purity.